Generally there are three different sources of gold(I) that can be used to catalyse a reaction. One source is gold(III) salts that are reduced in situ to gold(I) to catalyse a reaction. In such systems, it is not always clear to see whether gold(I) or gold(III) is the active species. Another source of gold(I) is the use of a simple inorganic gold(I) salt such as AuCl. Finally gold(I) can be used as a phosphine-stabilised cationic gold species, eg [Ph3PAu]+, with coordinating anions such as [TfO]−, [SbF6]−, [PF6]−. These species are generally formed in situ with the assistance of a silver salt containing the appropriate anion. This method of using a silver co-catalyst in the presence of a gold(I)phosphine is generally the method of choice seen in most gold(I) catalysed transformations.
Despite the use of a gold(I) phosphine and a silver salt as a co-catalyst being the most common catalytic system in gold(I) catalysis, there are some problems inherent in this methodology. Silver salts are known to be hygroscopic and light sensitive which can be an issue when weighing out the reagent, especially in small quantities. In 2005 Gagosz at al. documented some other possible drawbacks of using silver salts and suggest that the resultant phosphine gold(I) complexes may be unstable in solution (Org. Lett., 2005, 7, 4133-4136). Gagosz at al. developed a new phosphine gold(I) catalyst Ph3PAuNTf2 (Org. Lett., 2005, 7, 4133-4136), which was found to be air stable and easy to handle.
The first example of an asymmetric gold catalysed transformation, an aldol reaction between isocyanoacetates and aldehydes, was reported by Hayashi et al. in 1986 (J. Am. Chem. Soc., 1986, 108, 6405-6406). In general, most gold-based asymmetric catalysts use a chiral phosphine ligand to induce enantioselectivity in a transformation. In 2007 Toste et al. demonstrated that this approach is not always successful and reported that a chiral counterion could be used for an asymmetric hydroalkoxylation of allenols (Science, 2007, 317, 496-499). This example of chiral induction by a chiral ion pair shows that the chiral auxiliary does not necessarily need to be in the first coordination sphere of the gold(I) centre. The chiral ion pair can produce good enantioselectivity where traditional chiral ligands cannot. The combination of both strategies has also resulted in superior enantiomeric excesses compared to the use of only one of the two strategies.
There remains a need for further chiral ligands for gold and gold complexes containing the same that are active as asymmetric catalysts.